Foam plastics are known for their utility as lightweight yet strong materials that are environmentally stable. During production, the materials can be molded into essentially any shape, and thereby offer a wide range of utility, extending from construction and packaging materials to high performance materials for thermal insulation and impact resistance.
The polymerization and foaming are generally performed as two separate steps, enabling one to prepare the material ready for foaming in a manner which is convenient and economical for shipping and storage purposes. Foaming may then be done at the site of manufacture of the finished product, and may be tailored to any specific size or shape that the user requires.
To maintain product integrity and maximize the performance characteristics of the finished foam, a pre-foam is needed which will foam uniformly into a product capable of withstanding the mechanical stresses to be encountered during use. Structural integrity requires a uniform continuous cell structure within the foam, and optimum foams will be those having cells small in size and with a narrow size range. Wide variations in cell size and internal stresses in the overall cell structure render the foamed product vulnerable to rupture and the consequent loss of strength. In some foamable polymers, notably those produced by the copolymerization of (meth)acrylic acid and (meth)acrylonitrile, indications of faulty products are evident in the early stages of polymerization, as well as by a close inspection of the microstructure of the product. Premature precipitation, for example, in the liquid reaction mixture during polymerization frequently serves as an indication that nonhomogeneities will result. While the reason for the precipitation is not understood, it produces gradients in the microstructure as polymerization proceeds, resulting in pre-foams containing regions of non-homogeneity. Non-homogeneities are frequently evident due to their tendency to form undulations on the surface of a pre-foam piece rather than producing a piece with a completely flat surface. Non-homogeneous pre-foams are undesirable since they give rise to localized internal stresses upon foaming, which lead to weaknesses in the final structure and ultimately to rupture.
A number of disclosures in the patent literature present attempts to solve these problems. Schroeder et al., U.S. Pat. No. 3,627,711, addresses foams prepared from acrylamide or methacrylamide resins. The disclosure maintains that the inclusion of formamide as a foaming agent will prevent premature precipitation during the polymerization stage. The amount of formamide which can be used, however, limits the density range which can be achieved in the final product. Buchholz et al., U.S. Pat. No. 3,468,820, addresses foam plastics prepared from styrene polymers. The focus of this disclosure is the inclusion of a small amount of a polymer of which certain types are specified, to achieve control over cell size and uniformity upon foaming. Kanai et al., U.S. Pat. No. 3,489,700, discloses the improvement in water resistance obtained by copolymerizing acrylamide with acrylic acid esters. Unfortunately, the prefoam produced by this polymerization is rigid and susceptible to rupture upon expansion. Schroeder et al., U.S. Pat. No. 3,734,870 addresses foamed plastics formed from copolymers of (meth)acrylic acid and its amide or nitrile. The patent states that homogeneous copolymerization can be achieved by the inclusion of vinyl copolymers, silica gel or asbestos flour. Nevertheless, the polymerization is not truly homogeneous.